Eli5: What is the purpose of downshifting for a semi-truck and how does it work?

Question

I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?

In: 547

Answer 1

Slowing down truck takes energy. Usually all energy go to heating up brakes. Too much heat bad for brakes.

Downshift use engine to absorb energy too. Loud engine noise instead of hot brake.